Twisted waveguide and wireless device

ABSTRACT

H plane and E plane of a second rectangular waveguide element are inclined at an angle of 45° with respect to H plane and E plane of a first rectangular waveguide element. A connection element disposed between the first and second rectangular waveguide elements has an inner periphery that surrounds a central axis extending in a direction of electromagnetic-wave propagation. The inner periphery includes surfaces parallel to H plane and E plane of the first rectangular propagation path element, and these surfaces form a staircase such that abutting sections between the surfaces parallel to H plane and the surfaces parallel to E plane constitute projections. The staircase is inclined in a direction corresponding to a direction in which H plane of the second rectangular propagation path element is inclined. Accordingly, an electric field is concentrated in the projections of the connection element, and a plane of polarization of an electromagnetic wave propagating through the connection element is rotated from a plane of polarization in the first rectangular waveguide element towards a plane of polarization in the second rectangular waveguide element.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a national stage of PCT/JP2004/011243, filedAug. 5, 2004, which claims priority to Japanese application No.2003-347471, filed Oct. 6, 2003.

FIELD OF THE INVENTION

The present invention relates to a twisted waveguide that is capable ofrotating a plane of polarization of an electromagnetic wave propagatingthrough two rectangular propagation path elements.

BACKGROUND OF THE INVENTION

FIG. 14 illustrates a most-commonly-used conventional twisted waveguide,which is a rectangular waveguide having a twisted structure. Since arapid twisting of a twisted waveguide having such a structure is notallowed during its manufacturing process, the waveguide requires apredetermined length in the propagation direction of an electromagneticwave. Moreover, the waveguide also requires a large space in the jointportions. Japanese Unexamined Patent Application Publication No.62-23201 (“Patent Document 1 ”) discloses a structure for solving theseproblems. Specifically, FIG. 15 illustrates the structure of a twistedwaveguide according to Patent Document 1. In this twisted waveguide, asecond rectangular waveguide element 2 is attached in a manner such thatthe second rectangular waveguide element 2 is inclined at apredetermined angle with respect to a first rectangular waveguideelement 1. Furthermore, a resonant window or filter window 3 having atransmission center frequency as a predetermined frequency is disposedbetween the first rectangular propagation path element and the secondrectangular waveguide element 2 such that a plane of polarization isinclined at ½ of the predetermined angle mentioned above.

SUMMARY OF THE INVENTION

However, the structure shown in FIG. 15 is problematic in that theresonant window or filter window must have an extremely small dimensionin order to be used in a high frequency wave, such as in a W band (75 to110 GHz). This complicates the manufacturing process of the window, andmoreover, narrows the utilizable frequency range due to the utilizationof resonance.

Accordingly, it is an object of the present invention to solve theproblems mentioned above by providing a twisted waveguide having a wideutilizable frequency range without requiring a large dimension of aspace used for rotating a plane of polarization, and by providing awireless device equipped with such a twisted waveguide.

A twisted waveguide according to the present invention includes firstand second rectangular propagation path elements having different planesof polarization; and a connection element connecting the first andsecond rectangular propagation path elements together. The connectionelement has a fixed line length in a direction of electromagnetic-wavepropagation of the first and second rectangular propagation pathelements. The connection element includes projections projected inwardso as to face each other, the projections concentrating an electricfield of an electromagnetic wave entering from the first or secondrectangular propagation path element and rotating a plane ofpolarization of the electromagnetic wave propagating through theconnection element.

Furthermore, in the twisted waveguide according to the presentinvention, an inner periphery of the connection element surrounding acentral axis extending in the direction of electromagnetic-wavepropagation of the first and second rectangular propagation pathelements may include surfaces substantially parallel to H plane and Eplane of the first rectangular propagation path element. In this case,these surfaces form a staircase such that abutting sections between thesurfaces parallel to H plane and the surfaces parallel to E planeconstitute the projections. Moreover, the staircase is inclined in adirection corresponding to a direction in which H plane of the secondrectangular propagation path element is inclined.

Furthermore, in the twisted waveguide according to the presentinvention, the projections may include two projections provided at twopositions such that a plane extending between the two projections isinclined towards E plane of the second rectangular propagation pathelement with respect to E plane of the first rectangular propagationpath element.

Furthermore, in the twisted waveguide according to the presentinvention, the line length of the connection element in the direction ofelectromagnetic-wave propagation may be substantially ½ of a guidewavelength with respect to a frequency of an electromagnetic wave to bepropagated through the connection element.

Furthermore, in the twisted waveguide according to the presentinvention, the connection element may include a plurality of subelementsdisposed at multiple positions in the direction of electromagnetic-wavepropagation.

A wireless device according to the present invention includes thetwisted waveguide having one of the above structures; and an antennaconnected to one of the first and second rectangular propagation pathelements included in the twisted waveguide.

According to the present invention, a connection element disposedbetween first and second rectangular propagation path elements isprovided with projections projected inward so as to face each other.Thus, an electric field of an electromagnetic wave entering from thefirst or second rectangular propagation path element is concentrated inthe projections, and a plane of polarization of the electromagnetic wavepropagating through the connection element is rotated. Consequently, theplane of polarization is rotated in the connection element from thefirst rectangular propagation path element towards the secondrectangular propagation path element or from the second rectangularpropagation path element towards the first rectangular propagation pathelement. Since such a structure does not require a resonant window or afilter window shown in FIG. 15, a wide frequency range characteristiccan be achieved. Furthermore, according to this structure, since theplane of polarization is not rotated by a rectangular waveguide whoseoverall structure is twisted, the plane of polarization of anelectromagnetic wave can be rotated within a narrow space.

Furthermore, according to the present invention, an inner periphery ofthe connection element may be provided with surfaces substantiallyparallel to H plane and E plane of the first rectangular propagationpath element. Specifically, the surfaces form a staircase such thatabutting sections between the surfaces parallel to H plane and thesurfaces parallel to E plane constitute the projections. Moreover, thestaircase may be inclined in a direction corresponding to a direction inwhich H plane of the second rectangular propagation path element isinclined. Accordingly, each of the elements can be formed only of flatsurfaces and parallel surfaces, whereby the manufacturing process forthe first and second rectangular propagation path elements and theconnection element is simplified. This reduces the manufacturing cost,and therefore, contributes to the reduction of the overall cost.

Furthermore, according to the present invention, the projections mayinclude two projections such that a plane extending between the twoprojections may be inclined towards E plane of the second rectangularpropagation path element with respect to E plane of the firstrectangular propagation path element. Accordingly, the plane ofpolarization of the electromagnetic wave propagating through theconnection element can be rotated with only two projections, whereby theoverall structure is simplified. This further reduces the manufacturingcost.

Furthermore, according to the present invention, the dimension of theconnection element in the direction of electromagnetic-wave propagationmay be substantially ½ of a guide wavelength with respect to a frequencyof an electromagnetic wave to be propagated through the connectionelement. Thus, a consistency between the connection element and thefirst and second rectangular propagation path elements at the frequencycorresponding to the guide wavelength can be achieved. In other words,the reflection coefficient at the bordering section between the firstrectangular propagation path element and the connection element and thereflection coefficient at the bordering section between the secondrectangular propagation path element and the connection element havereversed polarities such that two reflection waves have opposite phasesand thus overlap. Accordingly, the two reflection waves counteract eachother, whereby a low reflection loss is achieved.

Furthermore, according to the present invention, the connection elementmay include a plurality of subelements disposed at multiple positions inthe direction of electromagnetic-wave propagation. Accordingly, evenwhen a rotation angle of a plane of polarization is not sufficientlyobtained at a first connection subelement, the total rotation angleobtained is large. Moreover, the structural differences at the borderingsections between the connection element and the first and secondrectangular propagation path elements can be reduced, thereby achievinga low reflection loss.

Furthermore, according to the present invention, a wireless device canbe readily provided in which the device can send or receive anelectromagnetic wave with a plane of polarization different from a planeof polarization in a propagation path through which a sending signal ora receiving signal propagates. For example, the device can send orreceive an electromagnetic wave whose plane of polarization is inclinedat a predetermined angle with respect to a horizontal plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a three-dimensionalconfiguration of an electromagnetic-wave propagation path of a twistedwaveguide according to a first embodiment of the present invention.

FIG. 2 FIGS. 2A, 2B and 2C are cross-sectional views each illustratingan element of the twisted waveguide of FIG. 1 and an electric-fielddistribution of an electromagnetic wave.

FIG. 3 illustrates reflection-loss-versus-frequency characteristics ofthe twisted waveguide of FIG. 1.

FIGS. 4A, and 4B are cross-sectional views each illustrating aconnection element of a twisted waveguide according to a second andthird embodiment of the present invention.

FIG. 5 is a perspective view illustrating a three-dimensionalconfiguration of an electromagnetic-wave propagation path of a twistedwaveguide according to a fourth embodiment of the present invention.

FIGS. 6A, 6B and 6C are cross-sectional views illustrating threestructural types of a connection element of a twisted waveguideaccording to a fifth embodiment of the present invention.

FIGS. 7A–7D are cross-sectional views of the elements of the twistedwaveguide according to the fourth embodiment.

FIG. 8 is a perspective view illustrating a three-dimensionalconfiguration of an electromagnetic-wave propagation path of a twistedwaveguide according to a sixth embodiment.

FIGS. 9A and 9B are cross-sectional views each illustrating a connectionelement of a twisted waveguide according to a seventh and eighthembodiment of the invention.

FIG. 10A is a three-dimensional configuration of an electromagnetic-wavepropagation path of a twisted waveguide according to a ninth embodiment,and FIGS. 10B–10E are cross-sectional views of the elements of FIG. 10A.

FIG. 11 illustrates S-parameter-versus-frequency characteristics of thetwisted waveguide of FIG. 10A.

FIGS. 12A and 12B show a primary radiator and a dielectric-lens antennaprovided in an extremely-high-frequency radar according to an tenthembodiment.

FIG. 13 is a block diagram illustrating a signal system of theextremely-high-frequency radar.

FIG. 14 is a perspective view of a conventional twisted waveguide.

FIG. 15 illustrates a twisted waveguide according to Patent Document 1.

REFERENCE NUMERALS SHOWN IN THE DRAWINGS

0 central axis

10 first rectangular waveguide element

20 second rectangular waveguide element

21 rectangular horn

30 connection element

31, 32 projection

40 dielectric lens

100, 101, 102 metal block

110 twisted waveguide

110′ primary radiator

R edge line

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A twisted waveguide according to a first embodiment will now bedescribed with reference to FIGS. 1 to 3.

FIG. 1 is a perspective view illustrating a three-dimensionalconfiguration of an inside (electromagnetic-wave propagation path) of atwisted waveguide 110. The twisted waveguide 110 includes a firstrectangular waveguide element 10 corresponding to a first rectangularpropagation path; a second rectangular waveguide element 20corresponding to a second rectangular propagation path element; and aconnection element 30 connecting the first rectangular waveguide element10 and the second retangular waveguide element 20. The first rectangularwaveguide element 10 and the second rectangular waveguide element 20propagate an electromagnetic wave of TE10 mode and each have an H planeextending longitudinally and an E plane extending laterally when viewedin cross section taken along a plane perpendicular to a direction ofelectromagnetic-wave propagation. The reference characters H in FIG. 1each indicate a surface parallel to a loop plane (H plane) of a magneticfield. Each reference character E indicates a surface parallel to aplane (E plane) extending parallel to a direction of an electric field.The first rectangular waveguide element 10, the second rectangularwaveguide element 20, and the connection element 30 have a commoncentral axis O (FIGS. 2A–2C) collinearly extending in the direction ofelectromagnetic-wave propagation.

If H plane of the first rectangular waveguide element 10 is parallel toa horizontal plane and E plane is parallel to a vertical line, H planeand E plane of the second rectangular waveguide element 20 are tilted atan angle of 45° about the central axis extending in the direction ofelectromagnetic-wave propagation.

The connection element 30 has a fixed line length in the direction ofelectromagnetic-wave propagation of the first and second rectangularwaveguide elements 10 and 20, and is capable of rotating a plane ofpolarization of an electromagnetic wave received from the firstrectangular waveguide element 10 or the second rectangular waveguideelement 20 so that a conversion can be performed between a plane ofpolarization of the first rectangular waveguide element 10 and a planeof polarization of the second rectangular waveguide element 20.

FIG. 2A through 2C are cross-sectional views of the elements shown inFIG. 1 each cross-sectional view is taken along a plane perpendicular tothe direction of electromagnetic-wave propagation. Similar to FIG. 1,only an internal space of the electromagnetic-wave propagation path isshown. Specifically, FIG. 2A is a cross-sectional view of the firstrectangular waveguide element 10, FIG. 2C is a cross-sectional view ofthe second rectangular waveguide element 20, and FIG. 2B is across-sectional view of the connection element 30. A pattern includingmultiple triangles in each drawing indicates an electric-fielddistribution of an electromagnetic wave of TE10 mode propagating throughthe twisted waveguide. In other words, the pointing direction of thetriangles of the pattern indicates the direction of the electric field,and the size and the density of the triangles of the pattern indicatethe magnitude of the electric field. In FIGS. 2A and 2C, each referencecharacter H indicates a surface parallel to H plane, and each referencecharacter E indicates a surface parallel to E plane. Referring to FIGS.2A FIGS. 2A and 2C, the electric field of TE10 mode extends in adirection parallel to E plane, and the intensity of the electric fieldis greater towards the center of each waveguide element. As describedabove, the first rectangular waveguide element 10, the secondrectangular waveguide element 20, and the connection element 30 have acommon central axis O collinearly extending in the direction ofelectromagnetic-wave propagation.

Referring to FIG. 2B, the connection element 30 is provided with a pairof projections 31 a, 32 a projected inward so as to face each other, anda pair of projections 31 b, 32 b also projected inward so as to faceeach other. The inner periphery of the connection element 30 includessurfaces Sh01, Sh02, Sh03, Sh11, Sh12, Sh13 which are parallel to Hplane of the first rectangular waveguide element 10; and surfaces Sv01,Sv02, Sv11, Sv12, Sv10, Sv20 which are parallel to E plane of the firstrectangular waveguide element 10. These surfaces parallel to H plane andthe surfaces parallel to E plane constitute a staircase-like structure.The direction of inclination of the staircase corresponds to thedirection in which H plane of the second rectangular waveguide element20 is inclined. In this embodiment, the staircase is inclined at anangle of 22.5°, which is substantially ½ of the angle of inclination ofH plane of the second rectangular waveguide element 20.

Abutting sections among the surfaces parallel to H plane and thesurfaces parallel to E plane of the first rectangular waveguide element10 constitute the projections 31 a, 32 a, 31 b, 32 b mentioned above.Consequently, the electric field is concentrated in these regions of theprojections 31 a, 32 a, 31 b, 32 b projected inward of the connectionelement 30. For this reason, a change in the direction of the electricfield is generated between the projections at the upper side and theprojections at the lower side of the connection element 30 in thedrawing. This tilts the plane of polarization of the electromagneticwave in the connection element 30, thereby rotating the plane ofpolarization of the electromagnetic wave propagating through theconnection element 30.

Referring to FIGS. 1, 2A, 2B and 2C, the waveguide element 10 and thewaveguide element 20 have different planes of polarization but have thesame cross-sectional structure. For this reason, a reflectioncoefficient as viewed from the side of the waveguide element 10 towardsthe connection element 30 and a reflection coefficient as viewed fromthe side of the waveguide element 20 towards the connection element 30can be made equal to each other in a relatively easy manner by adjustingthe height of the projections and the width of the projections in theconnection element 30. When the reflection coefficient viewed from theside of the waveguide element 10 towards the connection element 30 andthe reflection coefficient viewed from the side of the waveguide element20 towards the connection element 30 are equal to each other, thereflection coefficient viewed from the side of the waveguide element 10towards the connection element 30 and the reflection coefficient viewedfrom the side of the connection element 30 towards the waveguide element20 have the same magnitude with reversed polarities.

In this case, if the line length of the connection element 30 is set at½ of the guide wavelength, and supposing that an electromagnetic wavepropagates from the waveguide element 10 to the waveguide element 20, areflective wave at a bordering section between the waveguide element 10and the connection element 30 and a reflective wave at a borderingsection between the connection element 30 and the waveguide element 20overlap while being deviated from each other by one wavelength. Sincethe reflective waves of the reversed polarities overlap with each other,the reflective waves counteract each other.

FIG. 3 illustrates reflection-loss-versus-frequency characteristics ofthe twisted waveguide in a case where the two reflection coefficientsmentioned above have reversed polarities. The bold line in FIG. 3indicates a characteristic in a case where the line length of theconnection element is set at ½ of the guide wavelength at the designfrequency. On the other hand, the thin line corresponds to a comparativeexample and indicates a characteristic in a case where the line lengthis set at ¼ of the guide wavelength at the design frequency. If the linelength of the connection element is set at ¼ of the guide wavelength, alarge reflection loss of about −9 dB is caused due to reflectionsgenerated at the bordering planes between the first rectangularwaveguide element and the connection element and between the secondrectangular waveguide element and the connection element. On the otherhand, if the line length of the connection element 30 is set at ½ of theguide wavelength at the design frequency, the reflective wave generatedbetween the first rectangular waveguide element 10 and the connectionelement 30 and the reflective wave generated between the secondrectangular waveguide element 20 and the connection element 30counteract each other, whereby the reflection loss is minimized. Thedesign frequency of the twisted waveguide is 76.6 GHz at which thereflection loss is −60 dB as indicated by the bold line. Accordingly, anextremely low reflection-loss characteristic is achieved. Although thereflection loss increases as the frequency of the propagatingelectromagnetic wave deviates from the design frequency, a lowreflection-loss characteristic in which the reflection loss is −40 dB orless within a relatively wide frequency range of 76 to 77 GHz isachieved.

FIGS. 4A and 4B show twisted waveguides according to a secondembodiment, respectively, of the invention. FIGS. 4A and 4B arecross-sectional views of connection elements having different structurestaken along a plane perpendicular to the direction ofelectromagnetic-wave propagation, one of the connection elements beingincluded in the twisted waveguide. In contrast to the first embodimentshown in FIG. 1 provided with two pairs of projections (a total of fourprojections) projected inward to face each other, the second embodimentshown in FIG. 4A is provided with three pairs of projections (a total ofsix projections). Furthermore, the third embodiment shown in FIG. 4B isprovided with five pairs of projections (a total of 10 projections).Accordingly, the connection element 30 may be provided with a desirednumber of projections.

FIG. 5 illustrates a twisted waveguide according to a fourth embodiment.In this embodiment, H plane of the second rectangular waveguide element20 is inclined at an angle of 15° with respect to H plane of the firstrectangular waveguide element 10. This means that the connection element30 rotates the plane of polarization of an electromagnetic wavepropagating through the connection element 30 by an angle of 15°.Consequently, when the rotation angle is to be reduced, the angle ofinclination of the staircase portion of the connection element 30 ismade smaller, whereby the height of each step of the staircase isreduced. In contrast, if the rotation angle is to be increased, theangle of inclination of the staircase portion of the connection element30 is made larger, whereby the height of each step of the staircase isincreased.

A twisted waveguide according to a fifth embodiment will now bedescribed with reference to FIGS. 6A through 7D.

Each of the drawings mentioned above illustrates only the internalstructure of the electromagnetic-wave propagation path. Specifically,the twisted waveguide can be be formed by assembling together aplurality of metal blocks having grooves formed therein by, for example,cutting. FIGS. 6A–6C show three examples of such an assembly. Eachdiagram is a cross-sectional view of the connection element taken alonga plane perpendicular to the direction of electromagnetic-wavepropagation. A broken line in the diagrams corresponds to an attachmentplane (dividing plane) between metal blocks. The relationship betweenthe connection element and the first and second rectangular waveguideelements is the same as that shown in FIGS. 1 and 2. In each of FIGS. 6Aand 6C, a plane parallel to H plane of the first rectangular waveguideelement functions as a dividing plane. Specifically, in FIG. 6A, thedividing plane is set such that a groove formed in a metal block 101 hasa smaller number of inner surfaces therein. On the other hand, FIG. 6C,the dividing plane is set across the center of the connection elementsuch that grooves provided in upper and lower metal blocks 100, 101 aresymmetrical to each other.

In an example shown in FIG. 6B, planes parallel to E plane of the firstrectangular waveguide element function as dividing planes. Each dividingplane is set such that upper and lower projections of a correspondingpair facing each other is included in the same dividing plane. Accordingto this structure, the shape of grooves provided in metal blocks 100,101, and 102 is simplified, thereby achieving an easier machiningprocess.

FIG. 7A–7D are cross-sectional views of the elements including the firstand second rectangular waveguide elements in a case where the connectionelement has the structure shown in FIG. 6A. FIG. 7D is an explodedperspective view of this twisted waveguide. FIG. 7A is a cross-sectionalview of the first rectangular waveguide element 10, FIG. 7B is across-sectional view of the connection element 30, and FIG. 7C is across-sectional view of the second rectangular waveguide element 20.

An upper metal block 101 and a lower metal block 100 are each providedwith a groove for forming the first rectangular waveguide element 10 andthe connection element 30. The lower metal block 100 is integrallyprovided with a protrusion 102 in which the second rectangular waveguideelement 20 is provided. On the other hand, the upper metal block 101 isprovided with a recess which engages with this protrusion 102.

By setting the dividing plane in this manner, the shapes of the groovesprovided in the metal blocks 100, 101 for forming the first rectangularwaveguide element 10 and the connection element 30 are simplified,thereby achieving an easier manufacturing process.

FIG. 8 is a perspective view of a twisted waveguide according to a sixthembodiment of the present invention. Although the first and secondrectangular waveguide elements 10, 20 according to the embodiments shownin, for example, FIGS. 1 and 5 have the same size, these two elementsmay have different sizes. In the embodiment shown in FIG. 8, the firstrectangular waveguide element 10 is a W-band rectangular waveguideelement (75 to 110 GHz) having a preferred size of 2.54 mm×1.27 mm, andthe second rectangular waveguide element 20 is a V-band rectangularwaveguide element (50 to 75 GHz) having a preferred size of 3.10 mm×1.55mm.

When dealing with a signal of a 75-GHz band, a W-band rectangularwaveguide element and a V-band rectangular waveguide element may both beused. As shown in FIG. 8, the second rectangular waveguide element 20whose H plane is inclined in the direction of inclination of thestaircase of the connection element 30 is given a larger size than thefirst rectangular waveguide element 10 so that the structural differencebetween the connection element 30 and the second rectangular waveguideelement 20 is small. Thus, the reflection at the bordering sectionbetween these elements is maintained at a small amount.

FIGS. 9A and 9B show a main portion of a twisted waveguide according toa seventh and eighth, rspectively, of the present invention embodiment.In these embodiments, a pair of projections 31, 32 (a total of twoprojections) facing each other is provided. In FIGS. 9A and 9B thedirection of inclination of the staircase of the connection element 30corresponds to the direction in which H plane of the second rectangularwaveguide element is inclined such that a plane of polarization of anelectromagnetic wave can be rotated. In FIG. 9A, however, since the twoprojections 31, 32 face each other in a direction parallel to E plane ofthe first rectangular waveguide element, a region in which the electricfield is concentrated due to the two projections 31, 32 extends parallelto E plane of the first rectangular waveguide element. This results in alow ability for rotating the plane of polarization of an electromagneticwave propagating through the connection element 30 towards the plane ofpolarization in the second rectangular waveguide element. In contrast,in FIG. 9B, a plane extending between the projections 31, 32 facing eachother is inclined towards E plane of the second rectangular waveguideelement with respect to E plane of the first rectangular waveguideelement. Thus, the electric field that is concentrated in a regionbetween the two projections 31, 32 is tilted towards E plane of thesecond rectangular waveguide element. Accordingly, when theelectromagnetic wave entering from the first rectangular waveguideelement propagates through the connection element 30, theelectromagnetic wave is efficiently rotated towards E plane of thesecond rectangular waveguide element. According to this structureprovided with only a single pair of projections, a rotating effect forthe plane of polarization of the electromagnetic wave can still beachieved.

A twisted waveguide according to a ninth embodiment will now bedescribed with reference to FIGS. 10a through 10E and 11.

FIG. 10A is a perspective view illustrating a three-dimensionalconfiguration of the electromagnetic-wave propagation path. An edge lineR forming a hexahedron indicates an outline of assembled metal blocksthat form the waveguide elements. The first rectangular waveguideelement 10 and the second rectangular waveguide element 20 have theconnection element 30 disposed therebetween, and moreover, theconnection element 30 includes a first connection subelement 30 a and asecond connection subelement 30 b in this embodiment. FIG. 10B is across-sectional view of the first rectangular waveguide element 10,FIG.10C is a cross-sectional view of the first connection subelement 30 a,FIG. 10D is a cross-sectional view of the second connection subelement30 b, and FIG. 10E is a cross-sectional view of the second rectangularwaveguide element 20. The dimensions of the elements shown in thesediagrams are in millimeter units. Furthermore, the line length of thefirst connection subelement 30 a in the direction ofelectromagnetic-wave propagation is preferably 1.46 mm, and the linelength of the second connection subelement 30 b in the direction ofelectromagnetic-wave propagation is preferably 1.33 mm. The total linelength of the first and second connection subelements 30 a, 30 b is ½ ofa guide wavelength with respect to a frequency of an electromagneticwave to be propagated through the first and second connectionsubelements. Furthermore, the polarity of the reflection coefficient atthe bordering section between the first rectangular waveguide element 10and the first connection subelement 30 a is opposite to the polarity ofthe reflection coefficient at the bordering section between the secondrectangular waveguide element 20 and the second connection subelement 30b. Accordingly, two reflective waves generated at the two borderingsections counteract each other, whereby a low reflection-losscharacteristic can be achieved.

According to the connection element provided with two stages, therotation angle of a plane of polarization at each stage isadvantageously smaller, and moreover, the reflection loss at eachbordering section is also smaller. As a result, a twisted waveguideentirely having a low reflection-loss characteristic can be obtained.Moreover, since the total line length of the connection element is ½ ofthe guide wavelength, the entire structure does not need to be increasedin size.

Alternatively, each of the line lengths of the first and secondconnection subelements 30 a and 30 b may be set at ½ of a guidewavelength with respect to a frequency of an electromagnetic wave to bepropagated through the corresponding connection subelement. This furtherachieves a lower reflection-loss characteristic.

Each of the surfaces of the second rectangular waveguide element 20 isinclined at an angle of 45° with respect to the first rectangularwaveguide element 10. Accordingly, a staircase portion of the firstconnection subelement 30 a is inclined at an angle of approximately 15°,and a staircase portion of the second connection subelement 30 b isinclined at an angle of approximately 30°. Thus, the plane ofpolarization in each of the first and second connection subelements 30a, 30 b is rotated by approximately 22.5°, such that a total rotationangle of 45° is achieved.

FIG. 11 illustrates S-parameter-versus-frequency characteristics of thetwisted waveguide shown in FIG. 10A. According to a transmissiveproperty S21, a low loss characteristic of −0.5 dB or less is achievedover the range of 71 to 81 GHz or more. Moreover, a low reflectioncharacteristic of −25 dB or less is also achieved over the samefrequency range.

An extremely-high-frequency radar according to an tenth embodiment willnow be described with reference to FIGS. 12A, 12B and 13.

FIGS. 12A and 12B are perspective views of a dielectric-lens antennaprovided in the extremely-high-frequency radar. FIG. 12A shows a primaryradiator included in the dielectric-lens antenna. Here, a rectangularhorn 21 corresponds to the second rectangular propagation path elementaccording to the present invention. The connection element 30 includingthe first and second connection subelements 30 a, 30 b is disposedbetween the rectangular horn 21 and the first rectangular waveguideelement 10. The connection element 30 rotates a plane of polarization ofan electromagnetic wave propagating through the connection element 30.Accordingly, the first rectangular waveguide element 10, the connectionelement 30, and the rectangular horn 21 constitute a primary radiator110′.

FIG. 12B the structure of the dielectric-lens antenna. The rectangularhorn 21 of the primary radiator 110′ is disposed near a focal positionof a dielectric lens 40, and can be relatively shifted with respect tothe dielectric lens 40 so as to scan sending and receiving wave beams.Although a rectangular horn is provided in the primary radiator in thisembodiment, the primary radiator may alternatively be provided with, forexample, a cylindrical horn, a patch antenna, a slot antenna, or adielectric rod antenna.

FIG. 13 is a block diagram illustrating a signal system of theextremely-high-frequency radar provided with the dielectric-lensantenna. In FIG. 13, VC051 indicates a voltage controlled oscillatorwhich is provided with, for example, a varactor diode and one of a Gunndiode and an FET, and which sends an oscillation signal to a Lo-branchcoupler 52 via an NRD guide. The Lo-branch coupler 52 is a directionalcoupler including the NRD guide that extracts a portion of a sendingsignal as a local signal. A circulator 53 is an NRD-guide circulatorwhich sends the sending signal to the rectangular horn 21 of the primaryradiator in the dielectric-lens antenna, or transmits a receiving signalreceived from the rectangular horn 21 to a mixer 54. The mixer 54 mixesthe receiving signal from the circulator 53 and the local signaltogether so as to output a receiving signal Rx of an intermediatefrequency. A signal processing circuit, which is not shown, controls amechanism that positionally shifts the rectangular horn 21 of theprimary radiator 110′. Moreover, the signal processing circuit alsodetects the distance to a target and a relative speed based on therelationship between a modulating signal Tx of the VC051 and thereceiving signal Rx. As a transmission line other than the firstrectangular waveguide element 10 of the primary radiator 110′, an MSLmay be used instead of the NRD guide.

1. A twisted waveguide comprising: first and second rectangularpropagation path elements having different planes of polarization; and aconnection element connecting the first and second rectangularpropagation path elements, wherein the connection element has a fixedline length in a direction of electromagnetic-wave propagation of thefirst and second rectangular propagation path elements, and wherein theconnection element includes projections which project inward so as toface each other, the projections concentrating an electric field of anelectromagnetic wave entering from the first or second rectangularpropagation path element and rotating a plane of polarization of theelectromagnetic wave propagating through the connection element, andwherein an inner periphery of the connection element surrounding acentral axis extending in the direction of electromagnetic-wavepropagation of the first and second rectangular propagation pathelements includes surfaces substantially parallel to an H plane and an Eplane of the first rectangular propagation path element, said surfacesforming a staircase such that abutting sections between the surfacesparallel to the H plane and the surfaces parallel to the B plane formthe projections, the staircase being inclined in a directioncorresponding to a direction in which an H plane of the secondrectangular propagation path element is inclined.
 2. The twistedwaveguide according to claim 1, wherein the projections comprise twoprojections provided at two positions, wherein a plane extending betweenthe two projections is inclined towards an E plane of the secondrectangular propagation path element with respect to the E plane of thefirst rectangular propagation path element.
 3. The twisted waveguideaccording to claim 1, wherein the line length of the connection elementin the direction of electromagnetic-wave propagation is substantially ½of a guide wavelength with respect to a frequency of an electromagneticwave to be propagated through the connection element.
 4. The twistedwaveguide according to claim 1, wherein the connection element comprisesa plurality of subelements disposed at multiple positions in thedirection of electromagnetic-wave propagation.
 5. A wireless devicecomprising the twisted waveguide according to claim 1; and an antennaconnected to one of the first and second rectangular propagation pathelements included in the twisted waveguide.